1. Field of the Invention
The present invention relates to a semiconductor formed of a polycrystal silicon thin film, which is formed by crystallizing an amorphous silicon thin film on an insulator, and a thin-film transistor using that semiconductor.
2. Description of the Related Art
In recent years, a technique has been popularly researched where an amorphous silicon thin film is formed on an insulator such as a quartz substrate, allowed to grow into a solid-phase crystal (SPC) through a heating treatment or an annealing process due to the irradiation of a laser beam or an intense light to form a polycrystal silicon thin film.
A conventional general technique for obtaining a polycrystal silicon thin-film by allowing an amorphous silicon thin film to grow in a solid phase will be described below.
First, an amorphous silicon thin film having a thickness of 500 to 5000 xc3x85 is formed on a quartz substrate.
Thereafter, the amorphous silicon thin film thus formed is heated to 400 to 1100xc2x0 C. and subjected to an annealing process to make the amorphous silicon thin film grow into crystal. In this situation, a heater, infrared rays or the like is used as heating means.
The annealing process may be conducted by the irradiation of a laser beam or an intense light other than heating.
In the above-mentioned manner, a polycrystal silicon thin film is obtained.
The polycrystal silicon thin film thus obtained is used as an active silicon layer of the thin-film transistor (TFT), to thereby provide a thin-film transistor. As a result, a liquid-crystal display unit, image sensor or the like which is high in operation speed and in image quality is obtained using the above thin film transistor.
Up to now, the polycrystal silicon thin film obtained by annealing the amorphous silicon thin film has difficulty in lowering a defect density in crystal.
The thin-film transistor using the polycrystal silicon thin film thus formed as the active silicon film is hindered from realizing an improvement in various characteristics of the thin-film transistor, for example, the lowering of a threshold voltage (Vth), an increase in mobility, a decrease in a leak current (Ioff), etc., because the defect density in the active silicon layer is high.
The present invention has been made in view of the above circumstances, and therefore an object of this invention is to make a polycrystal silicon thin film obtained by annealing an amorphous silicon thin film low in defect density and high in quality.
Another object of the invention is to provide a thin-film transistor using a polycrystal silicon thin film obtained through an annealing process with a lowered threshold voltage (Vth) and leak current (Ioff) and an increased mobility.
In order to solve the above-mentioned problem, according to one aspect of the present invention, there is provided a method of manufacturing a semiconductor characterized in that, in polycrystallizing an amorphous silicon thin film formed on a substrate through an annealing process, said amorphous silicon thin film has a plane area of 1000 xcexcm2 or less.
In the above-mentioned method, the amorphous silicon thin film is preferably 1000 xc3x85 or more, more preferably 2000 xc3x85 to 10000 xc3x85 in thickness.
Also, according to another aspect of the present invention, there is provided a thin-film transistor having an active silicon film which is formed of a plurality of island-like regions arranged in parallel to each other, each of said island-like regions being formed of a polycrystal silicon thin film having a plane area of 1000 xcexcm2 or less.
In the thin-film transistor thus constituted, the island-like regions are formed of a polycrystal silicon thin film which is preferably 1000 xc3x85 or more, more preferably 2000 xc3x85 to 10000 xc3x85 in thickness.
Also, according to another aspect of the present invention, there is provided a method of manufacturing a thin-film transistor comprising the steps of: forming an amorphous silicon thin film on a substrate; processing said amorphous silicon thin film into a plurality of island-like regions having a plane area of 1000 xcexcm2 or less; polycrystallizing an amorphous silicon thin film that forms said island-like regions through an annealing process; and forming a thin-film transistor having at least one of said plurality of island-like regions as an active silicon layer.
In the above-mentioned method, the amorphous silicon thin film is preferably 1000 xc3x85 or more, more preferably 2000 xc3x85 to 10000 xc3x85 in thickness.
The present applicant has found that after the amorphous silicon thin film has been formed as island-like regions each has a plane area (an area viewed from an upper surface of the substrate) of 1000 xcexcm2 or less, it is subjected to an annealing process by heating or the irradiation of a laser light beam or an intense light to form a polycrystal silicon thin film with the result that a polycrystal silicon thin film which is low in defect density and high in quality is obtained.
FIG. 1 is a graph showing a relationship between a threshold voltage (Vth) and an area of the island-like regions of the polycrystal silicon thin-film transistor, in which each of the island-like regions is 1250 xc3x85 in thickness.
As shown in FIG. 1, it has been found that as the area of the island-like regions is small, the threshold voltage approaches 0 (zero) more in both of a p-channel and an n-channel so that the defect density is lowered.
In FIG. 1, there has been found that an extremely excellent crystalline property is obtained when the plane area of each of the island-like regions is 1000 xcexcm2 or less.
Also, when the plane area of each of the island-like regions is 1000 xcexcm2 or less, the island-like regions may be of a square, a rectangle or other shapes.
Further, when the island-like regions are 1 xcexcm2 or more in plane area, the semiconductor is satisfactorily available as a device, and also it can be readily manufactured by a usual technique.
On the other hand, in the case where the polycrystal silicon thin film is provided as the active silicon layer of the thin-film transistor, because an area of the island-like regions is limited in size, the thin-film transistor using that polycrystal thin film is also limited in size, which causes a limit to the performance of the thin-film transistor.
Under the above circumstances, the applicant has found that a plurality of each of the island-like regions, a plane area of which is 1000 xcexcm2 or less is arranged in parallel to each other as active silicon layers that constitute the source region, the drain region and the channel formation region of a thin-film transistor so as to increase a substantial channel width, thereby being capable of obtaining a polycrystal thin-film transistor which allows a sufficient amount of current to flow therein, which has the channel formation region low in defect density, and which is high in performance.
FIG. 3 shows an example of a plane shape of a thin-film transistor using a plurality of island-like regions as an active silicon layer.
In FIG. 3, a plurality of island-like regions 301 are arranged in parallel to each other to form an active silicon layer 305 of a thin-film transistor. On the island-like regions 301 are disposed a gate electrode 302, a source electrode 303 and a drain electrode 304 to form one thin-film transistor. Appropriate intervals between the respective island regions is several to several tens xcexcm. As the intervals are small, the plane area of the active silicon layer can be reduced more.
In the island-like region, as its plane area is reduced, the defect density is reduced more in a polycrystallized state, thereby being capable of reducing a leak current.
Also, the applicant has found that the amorphous silicon thin film is set to preferably 1000 xc3x85 or more, more preferably 2000 xc3x85 to 10000 xc3x85 in thickness, thereby lowering the defect density of the polycrystal silicon thin film obtained by crystallizing that amorphous silicon thin film.
FIG. 2 is a graph showing a relationship between the defect density of a polycrystal silicon thin film in a solid-phase growth and the thickness of an initial amorphous silicon thin film. In this case, a temperature of a solid-phase crystal (SPC) is 600xc2x0 C.
It has been found from FIG. 2 that as the thickness is made thin, the defect density is reduced more.
However, in crystallizing the thick initial amorphous silicon thin film through an annealing process, a stress of about 3xc3x9710xe2x88x929 dyn/cm2 due to a phase change is developed with the result that the polycrystal silicon thin film formed may be cracked.
Accordingly, when the polycrystal silicon thin film formed by crystallizing the thin amorphous silicon thin film is used as an active silicon layer that constitutes the channel formation region of the thin-film transistor as it is, this may cause the defect of the device or the lowering of the performance.
However, the applicant has found that even though the thickness of the amorphous silicon thin film is 1000 xc3x85 or more, more particularly 2000 xc3x85 to 10000 xc3x85, each of if the island-like regions formed of the amorphous silicon thin film are set to 1000 xcexcm2 or less in area and then annealed for crystallization, a polycrystal silicon thin film is obtained which has no cracks and is lower in the defect density.
Also, when the thickness of the amorphous silicon thin film becomes thicker than 10000 xc3x85, cracks are liable to occur.
According to the present invention, there can be obtained a polycrystal thin-film transistor that allows a sufficient amount of current to flow therein, has a channel formation region which is low in defect density, and is high in performance.
The thin-film transistor like this can reduce power consumption because a threshold voltage (Vth) and a leak current (Ioff) are lowered. Also, the thin-film transistor can operate at a high speed and allow a large current to flow therein because the mobility (xcexc) is increased.